Belt-type speed change units or variable transmission units are known, for the controlled driving of any appropriate member from a suitable drive means. For example, such a device may be used in a motor vehicle for driving, from the engine thereof, any auxiliary device, such as an alternator, water pump or fan.
Such a speed change device may comprise two pulleys, each comprising oppositely disposed side plates which are arranged coaxially with each other and one of which is fixed and the other of which is movable relative thereto by resilient return means. The side plates each have frustoconically extending portions defining a tapered groove, and the belt sits within said tapered groove so that when the movable side plate of one pulley moves towards the associated fixed side plate, the belt is urged radially outwardly relative to that pulley, riding radially outwardly on the inclined side faces of the groove. Conversely, the belt is displaced radially inwardly on the other pulley, being so permitted by the movable side plate of that pulley moving axially away from the fixed side plate, thus increasing the width between the inclined side faces of the groove of that pulley.
Various forms of such a device are known, but in some such devices the resilient return means associated with the movable side plate is formed by an annular member, usually referred to as a diaphragm. The diaphragm has a peripherally extending carrier portion which forms a Belleville washer-like arrangement and which bears against the movable side plate, and a central portion which is divided by radially extending slots into a plurality of radially extending fingers which bear against an abutment member, the abutment member is axially fitted with respect to the fixed side plate of the pulley.
One of the difficulties encountered in such a construction is that it is necessary to provide additional engagement means between the movable side plate and the diaphragm acting therein, either to provide for suitable centering of the diaphragm relative to the associated movable side plate or for also rotationally driving the side plate by means of the diaphragm. For example, such engagement means may comprise lugs which project radially from the diaphragm, around the edge of the above-mentioned peripheral portion and in continuity therewith, the lugs co-operating with complementary apertures or recesses provided for that purpose in the movable side plate.
However, such an arrangement suffers from a double disadvantage. Firstly, it requires a highly disadvantageous local modification in the annular form of the peripheral portion of the diaphragm, the disadvantage in this respect being that it is precisely this part of the diaphragm which is required to operate under substantial stresses, insofar as the peripheral portion has a spring-like action, in the manner of a Belleville washer. This local change in the peripheral portion of the diaphragm is inevitably likely to impair the operating performance and moreover its length of service life. It is in fact not uncommon to find in service that fracture lines and similar rupture beginnings occur on the peripheral portion of the diaphragm, for example extending radially thereacross, and such feature lines originate in the region of the angle along which the peripheral portion is connected to the lug which extends radially from the peripheral portion.
In addition, for a given diaphragm diameter, the above-mentioned lugs which radial extensions on the peripheral portion of the diaphragm will necessarily reduce the working diameter of the peripheral portion, in comparison with a diaphragm of the same overall diameter but without any radial lugs.
In another known construction, the engagement means operatively disposed between the movable side plate and the diaphragm are in the form of axially extending lugs or stubs fixed on the movable plate and engaged between the radially extending fingers of the diaphragm, thereby to co-operate with the circumferentially facing edges thereof. However, in this construction, contact between the engagement lugs or stubs and the radially extending fingers of the diaphragm is at a radial location thereof at which the axial displacement of the fingers relative to the movable plate is of a relatively substantial extent, in speed-changing operation of the pulley. This inevitably results in rapid wear at the positions where the fingers of the diaphragm bear against the lugs or stubs, and this results in the increasing development of a not inconsiderable amount of clearance between the fingers and the lugs or stubs, such clearance thus resulting in an increased amount of operating noise. In addition, besides the above-mentioned clearance, this construction also suffers from the disadvantages of considerable friction between the fingers and the lugs or stubs, which prevents free axial sliding movement of the movable side plate.